This application is a Continuation International Application No. PCT/JP01/02490, filed Mar. 27, 2001.
The present invention relates to a photocurable composition, a cured product comprising same, and a process for the preparation thereof. More particularly, the present invention relates to a photocurable composition comprising a small amount of an alicyclic skeleton-containing mono(meth)acrylate (hereinafter occasionally abbreviated as xe2x80x9cmono(meth)acrylatexe2x80x9d) incorporated in a combination of an alicyclic skeleton-containing bis(meth)acrylate (hereinafter occasionally abbreviated as xe2x80x9cbis(meth)acrylatexe2x80x9d) and a mercapto compound, a cured product obtained by the copolymerization thereof, and a process for producing the same.
A liquid crystal display panel which has heretofore been used comprises a glass plate incorporated therein as a substrate. However, since such a panel is limited in the reduction of density of glass and the enhancement of mechanical strength of glass, the recent requirement for reduction of weight and thickness cannot be met. Further, problems with enhancement of productivity have been pointed out from the standpoint of formability and workability. Thus, a panel comprising plastic as a substrate has been noted.
However, a plastic substrate has many problems with low birefringence suitable for liquid crystal display, heat resistance, mechanical strength, water absorption, product yield, etc.
Referring to low birefringence, a glass plate has a birefringence xcex94nxc2x7d of not greater than 1 nm while a plastic substrate has a birefringence xcex94nxc2x7d of not smaller than a few nanometers. Referring to heat resistance, a glass plate can withstand a liquid crystal cell assembly step at a temperature of not lower than 200xc2x0 C. while a plastic substrate is disadvantageous in that it undergoes deformation such as warpage due to insufficient heat resistance even if the assembly step is effected at a temperature as low as about 150xc2x0 C. Referring to water absorption, which raises no problems with glass plate, a plastic substrate is disadvantageous in that it undergoes dimensional deformation or causes the cell to undergo inability for display due to hygroscopic deformation when washed with water during the formation of cell.
As a means for solving these problems, Japanese Patent Laid-Open No. 1997-152510, etc. propose a low birefringence substrate obtained by subjecting a composition comprising bis(meth)acrylate and a polyfunctional mercapto compound having two or more thiol groups per molecule to curing by photopolymerization. The low birefringence substrate has a birefringence as low as not greater than 1 nm and a heat resistance as high as not lower than 150xc2x0 C. but has a deteriorated mechanical strength and hence a deterioration in flexibility and cracking resistance, which are inherent to plastic, due to the highly crosslinked high molecular structure. Accordingly, the sheet is damaged during the preparation of a plastic substrate having a thickness as relatively small as from about 0.05 mm to 3 mm or the plastic cell cracks during the falling of the display product, causing yield drop and hence adding to cost.
In order to improve the mechanical strength of the plastic substrate, a method (Japanese Patent Laid-Open No. 1998-77321) involving the incorporation of e.g., a thermoplastic resin in the photocurable composition which is a starting material constituting the substrate disclosed in the above cited publication and a method (Japanese Patent Laid-Open No. 1998-25621, Japanese Patent Laid-Open No. 1998-25624) involving the incorporation of other copolymerizable components in the photocurable composition are provided.
However, these improvements are disadvantageous in that since unpolymerizable components or components having different polymerization rates are incorporated, the properties inherent to photocurable composition are impaired, causing increase of birefringence, decrease of heat resistance and deterioration of water absorption and hence having adverse effects on production yield and cost reduction.
An object of the present invention is to provide a photocurable composition which, when photocured, provides a cured product that has an improved mechanical strength while maintaining its inherent properties such as low birefringence and high heat resistance, a cured product comprising same and a process for producing the same.
The inventors made extensive studies under these circumstances. As a result, it was found that the incorporation of a specific mono (meth) acrylate in the composition disclosed in Japanese Patent Laid Open No. 1997-152510 in a predetermined amount makes it possible to improve the mechanical strength thereof without impairing its inherent properties such as low birefringence and high heat resistance. The present invention has thus been worked out.
In other words, when a highly crosslinked high molecular structure comprises a non-crosslinked component having a structure and properties close to that of the high molecular substance incorporated therein, the mechanical strength thereof can be improved without increasing birefringence, decreasing heat resistance and deteriorating water absorption. As a result, the damage of the sheet can be lessened during the preparation of a plastic laminated product having a thickness as relatively small as from about 0.05 mm to 3 mm, preferably from about 0.1 mm to 1.5 mm.
This mono(meth)acrylate is produced as a precursor when bis (meth) acrylate is produced by the reaction of a diol form with (meth)acrylic acid or (meth)acrylic acid ester as starting materials and is a compound terminated by (meth)acryloyl group at one end thereof and by hydroxyl group at the other. Accordingly, by suspending the reaction in a stage before 100% progress of the reaction, the desired mixture of bis (meth) acrylate and mono (meth) acrylate can be easily obtained. This mono(meth)acrylate is analogous to bis(meth)acrylate in its chemical structure and thus is almost the same as bis(meth)acrylate in polymerization rate. Thus, this mono(meth)acrylate causes no increase of birefringence when cured by polymerization. Further, since this mono(meth)acrylate has an alicyclic skeleton similar to that of a specific bis (meth)acrylate, it has little effect on heat resistance and water absorption.
The essence of the present invention lies in:
1. A photocurable composition comprising the following components A, B and C (with the proviso that the proportion of the various components each are represented relative to 100 parts by weight of the sum of the weight of the components A and B):
Component A: 70 to 99 parts by weight of an alicyclic skeleton-containing bis(meth)acrylate represented by the general formula (I): 
xe2x80x83(wherein R1 and R2 each independently represent a hydrogen atom or methyl group; m represents 1 or 2; n represents 0 or 1; and p and q each independently represent 0, 1 or 2);
Component B: 1 to 30 parts by weight of an alicyclic skeleton-containing mono(meth)acrylate represented by the general formula (II): 
xe2x80x83(wherein R3 represents a hydrogen atom or methyl group; m represents 1 or 2; n represents 0 or 1; and r and s each independently represent 0, 1 or 2); and
Component C: 1 to 10 parts by weight of a mercapto compound having a functionality of at least two.
2. A cured product obtained by the copolymerization of a photocurable composition comprising the following components A, B and C (with the proviso that the proportion of the various components each are represented relative to 100 parts by weight of the sum of the weight of the components A and B) in the presence of a radical polymerization initiator:
Component A: 70 to 99 parts by weight of an alicyclic skeleton-containing bis(meth)acrylate represented by the general formula (I): 
xe2x80x83(wherein R1 and R2 each independently represent a hydrogen atom or methyl group; m represents 1 or 2; n represents 0 or 1; and p and q each independently represent 0, 1 or 2);
Component B: 1 to 30 parts by weight of an alicyclic skeleton-containing mono(meth)acrylate represented by the general formula (II): 
xe2x80x83(wherein R3 represents a hydrogen atom or methyl group; m represents 1 or 2; n represents 0 or 1; and r and s each independently represent 0, 1 or 2); and
Component C: 1 to 10 parts by weight of a mercapto compound having a functionality of at least two.
3. A process for producing a cured product which comprises the copolymerization of a photocurable composition comprising the following components A, B and C (with the proviso that the proportion of the various components each are represented relative to 100 parts by weight of the sum of the weight of the components A and B) in the presence of a radical polymerization initiator:
Component A: 70 to 99 parts by weight of an alicyclic skeleton-containing bis(meth)acrylate represented by the general formula (I): 
xe2x80x83(wherein R1 and R2 each independently represent a hydrogen atom or methyl group; m represents 1 or 2; n represents 0 or 1; and p and q each independently represent 0, 1 or 2);
Component B: 1 to 30 parts by weight of an alicyclic skeleton-containing mono(meth)acrylate represented by the general formula (II): 
xe2x80x83(wherein R3 represents a hydrogen atom or methyl group; m represents 1 or 2; n represents 0 or 1; and r and s each independently represent 0, 1 or 2); and
Component C: 1 to 10 parts by weight of a mercapto compound having a functionality of at least two.